1. Field of the Invention
This invention relates to the re-application of dye to a dye donor element of a thermal dye transfer system, and, more particularly, to the delivery of the dye from a reservoir through a membrane into a dye donor element. A variety of polyolefin or polyolefin containing materials were tested as membranes, and, in general, dye is permeable through the membrane. In addition, a variety of polymeric materials were tested as membranes, and, in general, the ability of dye to diffuse through a membrane increases as its Tg decreases.
2. Background Art
Color dye transfer thermal printers use a dye donor member which may be a sheet, but usually is in the form of a web advanced from a supply roll to a take-up roll. The dye donor member passes between a printhead and a dye receiver member. The thermal printhead comprises a linear array of resistive heat elements. In operation, the resistive heat elements of the printhead are selectively energized in accordance with data from a printhead control circuit. As a result, the image defined by the data from the printhead control circuit is placed on the receiver member.
A significant problem in this technology is that the dye donor members used to make the thermal prints are generally intended for single (one time) use. Thus, although the member has at least three times the area of the final print and contains enough dye to make a solid black image, only a small fraction of this dye is ever used.
After printing an image, the dye donor member cannot be easily reused, although this has been the subject of several patents. The primary reason that inhibits reuse of the dye donor members is that the dye transfer process is very sensitive to the concentration of dye in the donor layer. During the first printing operation, dye is selectively removed from the layer thus altering its concentration. In subsequent printings, regions of the donor member which had been previously imaged have a lower transfer efficiency than regions which were not imaged. This results in a ghost image appearing in subsequent prints.
The cost associated with having a single use donor ribbon is large because of the large area of ribbon required, as well as the large excess of dye coated on the donor member. While this technology is able to produce high quality continuous tone color prints, it is desired to provide an approach which has all of the good attributes of thermal dye transfer imaging but without the limitations associated with single use donor members.
Some work has been done by others to accomplish similar goals. For example, U.S. Pat. No. 5,286,521 discusses a reusable wax transfer ink donor ribbon. This process is intended to provide a dye donor ribbon that may be used to print more than one page before the ribbon is completely consumed. U.S. Pat. No. 4,661,393 describes a reusable ink ribbon, again for wax transfer printing. The ink ribbon contains fine inorganic particles and low melting waxy materials to assist in the repeated use of this ribbon. U.S. Pat. No. 5,137,382 discloses a printer device capable of re-inking a thermal transfer ribbon. However, again the technology is wax transfer rather than dye transfer. In the device, solid wax is melted and transferred using a roller onto the reusable transfer ribbon.
U.S. Pat. No. 5,334,574 describes a reusable dye donor ribbon for thermal dye transfer printing. This reusable ribbon has multiple layers containing dye which limit the diffusion of dye out of the donor sheet. This enables the ribbon to be used to make multiple prints. In addition, the ribbon may be run at a slower speed than the dye receiver sheet, enabling additional utilization. U.S. Pat. No. 5,118,657 describes a multiple use thermal dye transfer ink ribbon. This ribbon has a high concentration dye layer on the bottom and a low concentration dye layer on the top. The low concentration dye layer meters or controls dye transfer out of the ribbon. This enables the ribbon to be used multiple times. U.S. Pat. No. 5,043,318 is another example of a thermal dye transfer ribbon which can be used multiple times.
Accordingly, there is no prior art known to us which directly relates to the concept of the re-application of dye to a dye donor ribbon.
It is a feature of the present invention to provide a reservoir containing a supply of dye which is transferred from the reservoir to the dye donor element by diffusion of dye into the dye donor element.
It is another feature of the present invention to provide a reservoir containing a supply of dye, the reservoir having a diffusion controlled permeation membrane through which dye is delivered to the dye donor element.
It is still another feature of the present invention to provide a diffusion controlled permeation membrane which inhibits diffusion of an optional binder, whereby the dye partitions or diffuses between the reservoir and the dye donor element but the binder does not. The reservoir may also include a porous sub-layer covered by the diffusion controlled permeation membrane through which dye is delivered from the sub-layer to the dye donor element. Further, the reservoir may be a roller with the membrane forming a cylindrical cover for the sub-layer.
According to the invention, dye is thermally transferred from a reservoir to the depleted donor patch. The dye and a binder are contained in the reservoir. The reservoir is covered with a diffusion controlled permeation membrane. With the addition of heat, dye diffuses through the membrane and is delivered to the donor patch. The dye partitions between the reservoir and the donor patch reestablishing the original dye concentration.
Accordingly, the invention resides in an apparatus for re-applying dye to a dye donor element of a dye transfer thermal printer, the apparatus comprising a thermal dye donor element; a printing station at which dye is image-wise transferred from the dye donor element to a receiver medium, at least partially depleting the dye donor element of dye; a reservoir containing dye and an optional binder; and means for transferring dye from the reservoir to the dye donor element by separating the dye from the binder by diffusion of dye into the dye donor element wherein the reservoir includes a diffusion controlled permeation membrane through which dye is delivered to the dye donor element and wherein said membrane comprises a linear, branched and/or crosslinked polymer or copolymer.
The invention, and its objects and advantages, will become more apparent in the detailed description of the preferred embodiments presented below.